Currently, with a custom SMS or Email provider, bulk actions do not work (i.e, from the contact page you cannot bulk send an email or SMS using a custom provider. Also, email/SMS reply triggers do not work currently for custom providers.
With the newest API update, users can bring their own Email and SMS provider (including Gmail, which has been a highly requested feature for a while), but they cannot use all of the native functions of the CRM with the custom provider because it won't recognize it.
This would be a minimally labor-intensive task that could be accomplished quickly and would greatly benefit the entire community!